We often have code that does the following operation:

if (str.startsWith(x)) {
    doSomethingWith(str.first(x.size()));
    str.slice(x.size());
}

ie. it checks for a prefix, x, and consumes it before the next step. Ditto endsWith() and last()/chop().

This works fine in a world where everything is UTF-16. It even still works if you throw L1 into the mix, because L1 is a fixed-width encoding and those Unicode characters that require more than one UTF-16 code point never match L1, so lengths are still consistent between L1 and UTF-16.

But if one of str xor x is UTF-8, this code breaks apart.

This is an example of a function that throws away useful (and hard to calculate) information without returning it. In the implementation of startsWith, the equivalent end of x in *this is well-known. But instead of returning this information as a prefix string-view, we throw it away.

So add a function that returns this information, and is otherwise a drop-in replacement for startsWith()/endsWith(). That means the new function returns an optional-like structure that, if engaged, contains the subset of *this that matched as a view.

Acceptance criteria:

• decide whether to extend the existing beginsWith()/endsWith() or to add a new function
• add functionality to all strings and views that have startsWith()/endsWith()
• document it
• test it
• port some users in Qt
  • (say at least one of prefix/suffix for each of the classes that get the new functionality